Tilting of the Bergell Pluton and central Lepontine area: Combined evidence from paleomagnetic, structural and petrological data

Paleomagnetic data from two undeformed sites in the steep N-dipping tonalite rail of the Oligocene Bergell pluton indicate a mean natural remanent magnetization (NRM) with declination D = 303.5 degrees and inclination I = 38 degrees. Restoration of the measured NRM direction to the NRM direction of stable Europe in the Oligocene is achieved by three different rotations corresponding to three distinct post-crystallization deformation events. 1) 25 degrees dextral rotation around a vertical axis in order to restore the sinistral block rotation suffered by each segment of the tonalite tail during dextral shearing along Insubric Riedel faults. 2) 20 degrees dextral rotation (looking westward) around an horizontal E-W striking axis in order to restore the rotation suffered by the tonalite tail during backthrusting along the Insubric Line. 3) 11 degrees sinistral rotation (looking northward) around an horizontal N-S striking axis in order to restore the effect of eastward tilting of the Bergell pluton. The similar inclinations of the Oligocene NRM of stable Europe and that of the Bergell tonalite after rotation of only 20 degrees around an E-W horizontal axis indicate that the Bergell tonalite tail was in a steep orientation at the time it passed through its solidus. Moreover, the concordant contact of the tonalite rail with the enclosing rocks of the Southern Steep Belt of the Central Alps implies that the Southern Steep Belt already existed when the tonalite tail reached its solidus. It is suggested that vertical uplift of the western Lepontine induced eastward tilting of the Central Lepontine area. This rotation around a N-S striking horizontal axis explains deviation of paleomagnetic directions (Heller 1980) from the orientation of stable Europe in the Oligocene.